The invention relates to a friction clutch and to a belt retractor incorporating such a friction clutch.
Known friction clutches have as friction member a metal disc which is arranged between two clutch plates and is provided on both sides with a clutch lining. The clutch lining ensures on the one hand a high frictional force to the clutch plates and on the other hand is designed such that it can deal with the stresses of pressure and temperature during operation. The friction member is hence always composed of several parts of different material. In order to ensure the necessary contact pressure and hence a sufficient frictional force, the clutch plates and the friction member are pressed together by means of a separate spring.
In known belt retractors, a belt force limitation is achieved in that in the force flow path between the belt spool and the locking disc a torsion rod is arranged, which is twisted on exceeding a predetermined torque and in so doing is plastically deformed. A controlled torque limitation is only possible here once and in one direction.
The object of the invention is to provide a friction clutch and a belt retractor with torque limitation, which operate in two directions in a force-limiting manner and at the same time are constructed simply and are able to be manufactured at a favorable cost.
This is achieved in a friction clutch which comprises two clutch faces lying opposite each other and a friction member arranged between the clutch faces. The friction member is constructed as a flat spring part having shaped sections, the shaped sections lying at least partially against at least one of the clutch faces. Such a friction clutch is able to be produced particularly simply and at a favorable cost, because the necessary contact pressure between friction member and clutch faces is applied by the friction member itself, because this is constructed as a spring part. A separate pressure spring can therefore be dispensed with. The friction member can be produced for example in a simple manner as a sheet metal stamped member at low cost. The torque which is able to be transferred through the friction clutch is codetermined substantially by the elastic force of the spring part.
In a further embodiment of the invention, the shaped sections have a cross-section in the form of a circular arc as seen in a direction parallel to the clutch faces. Good elastic characteristics are achieved with a cross-section in the form of a circular arc. In tangential direction to the cross-section of the shaped sections in the form of a circular arc, the transferable friction force is, moreover, independent of direction.
Advantageously, the spring part is constructed as a corrugated band. Such a corrugated band is constructed in a simple manner and is suitable for arrangement between concentric clutch faces. Both the shaped sections, lying partially against the first clutch face and also the shaped sections of the spring part lying partially against the second clutch face then have a cross-section in the form of a circular arc. These measures also allow the friction member to consist of one single part.
It is likewise advantageous if the spring part is constructed as a cylinder with elastic shaped sections. Here, also, the friction member consists of a single part and is able to be mounted in a particularly simple manner between two concentric clutch faces, because the cylinder has a certain inherent stability and can thus be easily pressed in between the clutch faces.
In a further advantageous embodiment, the spring part is constructed as a plate with elastic shaped sections. Such an embodiment permits the arrangement between two clutch plates. The spring part likewise consists of a single part and can be produced in a particularly simple manner as a sheet metal stamped part.
According to the invention, there is also provided a belt retractor comprising a belt spool rotatably mounted in a frame, a locking mechanism for the belt spool with at least one locking disc connected to the belt spool, and a torque limiting member arranged in a force flow path between the belt spool and the locking disc, in which the torque limiting member is constructed as a friction clutch. Such a belt retractor acts in a force-limiting manner in two directions, because the friction clutch limits the torque in two directions. Hence, peak values of the belt force can be avoided both with the locking of the belt spool in a vehicle-sensitive or belt webbing-sensitive manner and also during operation of a belt tensioner engaging on the belt spool via the friction clutch. In contrast to conventional belt retractors with torsion rods as torque limiting member, the belt retractor according to the invention can also be re-used after a torque limitation has been occurred.
Advantageously, a first clutch plate is provided which is connected non-rotatably with the belt spool and presents a first clutch face and a second clutch plate is provided which is connected non-rotatably with the locking disc and presents a second clutch face, a plate being arranged between the first and second clutch faces which has elastic shaped sections. These measures make possible a space-saving construction of the belt retractor, because the friction clutch is constructed so as to be flat and extends in radial direction. The friction clutch can be arranged for example outside the frame of the belt retractor on the belt spool axle.
An advantageous embodiment is also produced in that a first cylinder face is provided which is connected non-rotatably with the belt spool and presents the first clutch face and a second cylinder face is provided which is connected non-rotatably with the locking disc and presents the second clutch face, a cylinder being arranged between the first and second clutch faces which has elastic shaped sections. In such a belt retractor, the spring part is stressed uniformly, because the same relative speed occurs everywhere between the concentric clutch faces.
In a further embodiment of the invention, provision is finally made that the first clutch face is formed by a side face of an extension of a belt spool axle non-rotatably connected to the belt spool, and the second clutch face is formed by an inner face of a hollow cylinder non-rotatably connected to the locking disc. To realize the friction clutch, such a belt retractor requires only a small effort as regards construction, because it is merely necessary to alter already existing parts, namely the belt spool axle and the locking disc, and also the spring part. The belt spool axle merely has to be provided with an extension, extending for example outside the frame, and a hollow cylinder, concentric to the belt spool axle, must be attached to the locking disc, The spring part is then pushed onto the extension of the belt spool axle and into the hollow cylinder of the locking disc.